bald spot on head?

One of our two-month-old chicks (a Cuckoo Maran) had a tiny bald spot on her head when we got her at two weeks old. I figured it was a tiny scratch or maybe a congenital abnormality. Unfortunately, as she grows, the spot seems to be growing proportionally with her. It's now about 3/4 the size of a dime (but ovalish and irregular). Can anyone shed any light on this? Even if it doesn't get much bigger, I'm concerned about sunburn. It's slightly red around the edges now as well, and this is new. All the other 5 girls seem totally fine, not itchy, have a very clean, shady, coop, etc.
Thanks!
Tuli